Disorders of Consciousness

G. Bryan Young, Eelco F. M. Wijdicks

• Publisher: Elsevier
• Year: 2008
• Print ISBN: 9780444518958
• eBook ISBN: 9780080569857
• Topic: Medizin
• Subtopic: Neurologie

This volume encompasses a variety of topics pertaining to patients with altered levels of consciousness, including valuable differences between disorders. Neurologists, researchers, and clinicians will find a comprehensive accounting of the distinctions between disorders that cause these altered states. Beginning with basic concepts of consciousness and neurobiology, this handbook progresses into more targeted and complex areas of discussion, including important technological advancements that have occurred in neuroimaging.Neurologists who are frequently called upon for prognostication and to guide management of patients with these disorders will find invaluable information, including chapters discussing comatose states in children and pregnant women, encephalopathic patients, nutrition disorders, and vegetative and minimally conscious states. In addition, chapters devoted to philosophical backgrounds and ethical implications involving patients with impaired consciousness, are thoroughly presented.* A comprehensive reference guide that progresses from basic concepts of consciousness and neurobiology to targeted, complex topics of discussion* Invaluable information on new technological advancements that have occurred in neuroimaging, thus furthering diagnostic and treatment modalities* Philosophical and ethical implications involving the relationship between clinicians and patients with impaired levels of consciousness

Section 1

Theory and practice

Chapter 1

Consciousness: Concepts, Neurobiology, Terminology of Impairments, Theoretical Models and Philosophical Background

Adam Zeman* Peninsula Medical School, Exeter, UK
^* Correspondence to: Adam Zeman, Professor of Cognitive and Behavioural Neurology, Peninsula Medical School, Barrack Road, Exeter EX2 5DW, UK. Tel: [phone] (direct); [phone] (NHS secretary); [phone] (academic secretary). email address: [email protected]

Publisher Summary

The problem of consciousness, the mind–brain question in its modern disguise, is ancient and persistent. The chapter outlines the principal senses of consciousness, particularly those relevant to science and medicine. Self-consciousness implies more than an ability to behave differently towards self and other and it requires a representation of self and other. The current knowledge of the neurobiology of consciousness is summarized in its two key senses: wakefulness and awareness. Functional imaging studies have made it possible to explore the anatomy and physiology of sleep and arousal in the healthy human brain. Variations in the level of arousal during wakefulness also correlate with levels of activity in the structures of midbrain and thalamus that regulate conscious states. The chapter outlines the principal pathologies of wakefulness and awareness. Pathologies of conscious state can be classified with respect to duration, underlying cause, or clinical features. The chapter describes currently prevailing, overarching, models and theories of consciousness. The theories emphasize the anatomical organization of the brain networks and the nature of the psychological processes involved in consciousness. Consciousness plays a role in the control of behavior, specifically in circumstances that involve novel challenges or unpredictable events. The chapter also presents a survey of contemporary philosophical views of the relationship between mind and brain.

1.1 Concepts

1.1.1 Consciousness in context

There has been an extraordinary flowering of interest in the subject of consciousness since the topic was addressed in broad terms in this Handbook by J. A. M. Frederiks in 1969 (Frederiks, 1969). This flourishing is attested by a steady stream of publications, both technical and popular (Jasper et al., 1998; Damasio, 2000; Zeman, 2002; Dehaene and Naccache, 2003; Koch, 2004; Laureys, 2005), the emergence of an association dedicated to the study of the topic (Association for the Scientific Study of Consciousness), and a busy schedule of related scientific meetings (e.g., the annual meetings of the ASSC, the two-yearly Tucson meetings, Towards a Science of Consciousness).

It is worth asking why the subject has prospered so mightily in recent years. Several interrelated developments have contributed. First, experimental and clinical advances, in cognitive neuroscience and neuropsychology, are revealing ever more exquisite correlations between features of experience and events in the brain. The advent of functional imaging, in particular, is enabling us to see something of what happens in the human brain during experience – and in its absence, for example during coma. Second, the realization that unconscious neural processes are ubiquitous in the brain, and often affect our behavior, has helped to throw the topic of conscious processes into relief. Third, the design of increasingly sophisticated forms of artificial intelligence raises the possibility that we may become able to create conscious systems: what once was science fiction may soon be science fact. The fourth reason for the current fascination with the topic of consciousness is the most profound: the cartesian separation of brain and mind is untenable, both intellectually and in clinical practice. But what is the true nature of the opaque relationship between mind and matter? How does the brain give rise to consciousness?

This central ‘problem of consciousness’, the mind–brain question in its modern disguise, is ancient and persistent. The dichotomy between mind and brain is reflected in the apparent disconnection between work in the two great intellectual domains of relevance to the study of consciousness – the Humanities, focusing on the experiences of subjects, and the Sciences, highlighting processes in systems. Within medicine, this intellectual divide is mirrored in the historical separation of psychiatry and neurology. The hope of contemporary students of consciousness is that progress in solving the central problem of how the brain gives rise to consciousness will build a trustworthy bridge between mind and brain, explaining how experience can be at once real, functional and rooted in our physical existence (Zeman, 2001). Beyond question, the scientific world view will be incomplete until it incorporates a clearer understanding of our subjectivity.

Like the mind–body problem itself, the notion that the brain is the source of consciousness is very ancient, as revealed by this famous and prescient passage from Hippocrates’ essay ‘On the Sacred Disease’ (Jones, 1923):

Men ought to know that from the brain, and from the brain only, arise our pleasures, joys, laughter and jests, as well as our sorrows, pains, griefs and tears. Through it … we think, see, hear, and distinguish the ugly from the beautiful, the bad from the good, the pleasant from the unpleasant … sleeplessness, inopportune mistakes, aimless anxieties, absent-mindedness, and acts that are contrary to habit. These things that we suffer all come from the brain.

Yet arguably progress in understanding exactly how experience ‘arises’ from the brain has been disappointingly slow. Writing 2500 years after Hippocrates, E.O. Wilson identifies the problem as a central issue for contemporary science (Wilson, 1998): ‘the master unsolved problem of biology: how the hundred billion nerve cells of the brain work together to create consciousness’.

Granted that science has in fact made great strides in revealing the physical basis of consciousness over the past century, as outlined in the following sections, and yet the ‘master problem’ appears to be ‘unsolved’, one has to wonder whether part of the problem here may be conceptual rather than empirical. The philosopher Leibniz voiced an idea of this kind in his Monadology, in a passage that invites us to imagine walking into the midst of an artificial brain (Leibniz, 1714):

Perception and that which depend on it are inexplicable by mechanical causes, that is by figures and motions. And supposing there were a machine so constructed as to think, feel and have perception, we could conceive of it as enlarged and yet preserving the same proportions, so that we might enter into it as into a mill. And this granted, we should only find on visiting it, pieces which push against one another, but never anything by which to explain perception.

Leibniz is suggesting here that no mechanistic theory can ever, in principle, provide a really satisfying explanation of consciousness. Many people have this intuition. What is its source?

For better or worse, the concept of consciousness has been shaped by our cultural, religious and philosophical history. Certainly ‘consciousness’, as it is generally understood, is far from being a simple scientific variable. Surveys suggest that the predominant notion of consciousness in our culture is of a private, invisible, immaterial process, inaccessible to the standard observational methods of science. On such an assumption it is indeed hard to see how science could truly fathom the relationship between consciousness and the brain.

However, it may well be that scientific advances, and philosophical analysis, will gradually modify both the scientific and the popular concepts of consciousness. There are strong reasons, discussed below, for doubting that our grasp of the contents and the nature of experience is as firm as we usually take it to be. When we look back from the terminus of the quest for consciousness we may see our point of departure in an entirely new light.

The aims of this introductory chapter are: 1) to outline the principal senses of consciousness, particularly those relevant to science and medicine; 2) to summarize current knowledge of the neurobiology of consciousness in its two key senses of wakefulness and awareness; 3) to relate this to the principal pathologies of wakefulness and awareness; 4) to sketch the currently prevailing, overarching, models and theories of consciousness; and 5) finally to return to the philosophical issues just touched on, with a succinct survey of contemporary philosophical views of the relationship between mind and brain.

1.1.2 Senses of consciousness and self-consciousness

Part of the problem of consciousness is semantic: it is an ambiguous term, with several strands of meaning. This is all the more true of ‘self-consciousness’. I shall briefly discuss the etymology and principal senses of these words.

1.1.2.1 The etymology of ‘consciousness’ and ‘conscience’

The word ‘consciousness’ has its Latin root in conscio, formed by the coalescence of cum, meaning ‘with’, and scio, meaning ‘know’ (Lewis, 1960). In its original Latin sense, to be conscious of something was to share knowledge of it with someone else or, metaphorically, to share it with oneself. The knowledge in question was often of something secret or shameful, the source of a bad conscientia, a bad conscience. This meaning of conscientia, implying knowledge shared, has been referred to as its strong or narrow sense. A weakened, or broad, sense coexisted with it in which conscientia meant, simply, knowledge. All three senses – of knowledge shared with another, knowledge shared with oneself and, simply, knowledge – entered the English language with ‘conscience’, the first English derivative of conscientia. The words ‘conscious’ and ‘consciousness’ first appear early in the 17th century, followed by ‘self-conscious’ and ‘self-consciousness’.

1.1.2.2 What is meant by ‘conscious’?

The Oxford English Dictionary distinguishes 12 senses of ‘conscious’ and eight of ‘consciousness’. Con...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright page
• Handbook of Clinical Neurology 3rd Series
• Foreword
• List of Contributors Volume 90
• Section 1: Theory and practice
• Section 2: Syndromes of impaired consciousness
• Section 3: Specific entities
• Section 4: Management issues
• Subject Index